Chucks for clamping a shank portion of a rotatable machining tool, such as a milling cutter or a drill, are well known within the machining industry, especially for machining work pieces of steel or other metals, but also for wood and plastics. Such a chuck is in its turn mounted in a suitably designed machine for rotatably driving of the chuck. Usually, the attachment of the tool is accomplished purely by frictional clamping, such that clamping surfaces within a clamping bore of the chuck are tightened around the shank portion. The clamping force can be generated in different ways, such as mechanical displacement of clamping jaws inside the clamping bore, hydraulic deflection of a clamping insert or collet inside the clamping bore or thermal crimping of the chuck body around the shank portion.
One problem with a purely frictional clamping of the shank portion of the machining tool is that, despite applying a maximal possible clamping force, the machining tool may during operation tend to rotate and/or be displaced in the axial direction in relation to the chuck. The reason for this is usually to be found in the combination of the vibrations to which the machining tool is exposed during the machining operation, especially when the machining is performed at high rotational speed, and the tangential and axial forces acting on the tool during operation.
The above problem can be overcome by a frictional clamping chuck and a milling cutter as disclosed in U.S. Pat. No. 8,505,893, in which the inner end portion of the milling cutter is provided with helical or L-shaped locking grooves, which are adapted to engage with balls or pin-shaped locking elements. In an alternative embodiment in that document, the circumferential surface at the inner end portion of the shank of the machining tool is provided with a male thread, which is in engagement with a female thread in the inner portion of the bore. With a chuck formed in this way, the machining tool will, due to the locking grooves or threads, be prevented from rotation, in at least one direction, as well as be prevented from being pulled out in the axial direction from the bore.
However, there are also several drawbacks with a chuck and machining tool designed as in U.S. Pat. No. 8,505,893. Since the envelope surface of the tool shank is provided with some kind of threads or locking grooves, the clamping zone between the chuck and the tool will be negatively affected in one way or the other. For example the clamping portion of a hydraulic expansion chuck is not well suited to clamping around threads or grooves, since this could cause permanent deformation of the hydraulically deflectable sleeve inside the chuck. Also, the ball- or pin-shaped locking elements cannot be allowed to penetrate through the channels and spaces for hydraulic fluid between the deflectable collet and the chuck body, nor can a female thread be formed in the deflectable collet. Accordingly, in this case the clamping portion of the chuck has to be made shorter, which will render the frictional clamping forces acting on the tool lower, or alternatively to make the tool shank and the chuck bore longer, which will increase the quantity of material for manufacturing the tool, and hence increase the costs, and take up more space inside the chuck, which accordingly also has to be made longer.
For a mechanically operated draw-in collet chuck, the ball- or pin-shaped locking elements will complicate the design and disturb the function of the clamping jaws and a female thread cannot be formed in the clamping jaws. Accordingly, the clamping zone between the clamping jaws and the tool shank will be reduced due to the threads or locking grooves with lowered frictional clamping force as a result, unless this is compensated by an extended tool shank and chuck bore. Also, in case of a thermal expansion chuck the thread or the locking grooves may reduce the frictional clamping force, which may have to be compensated by an extended tool shank and chuck bore. Moreover, should the machining tool break such that only the tool shank is left inside the chuck bore, the remaining tool shank is very difficult to remove from the chuck bore.
In addition to the above mentioned drawbacks, the embodiment having a male thread on the envelope surface of the tool shank and a female thread inside the chuck bore will be difficult and hence costly to produce, since the female thread has to be shaped in the bottom of a rather narrow bore.